Hybrid drive device

ABSTRACT

A hybrid drive device comprises an internal combustion engine and an energy converter adapted to co-operate with an output shaft of the internal combustion engine and which comprises an electric machine. The electric machine includes a first rotor connected to the output shaft of the internal combustion engine and a second rotor connected to a drive shaft. The two rotors are arranged with surfaces for co-operation by transmitting power through magnetism directed substantially in the direction of the axis of rotation of the rotors for substantially axial magnetic flux between the rotors.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a hybrid drive device “Electricmachine” is defined in such a way that it is also comprised that aco-operation by transmitting power through magnetism may take placebetween more than two parts movable with respect to each other, whichwould in the technical field sometimes be considered as several electricmachines, but it is here considered as one single electric machine.Thus, a so called double rotor machine with a stator is for example tobe regarded as an electric machine falling under the definitionaccording to the invention, although such a machine is sometimesconsidered to be two electric machines connected in series.

For a hybrid drive device, especially such arranged in the enginecompartments of a motor vehicle, the space available for said energyconverter is normally strongly restricted, especially in axialdirection. The engine compartment of motor vehicles has normally apredetermined maximum volume, which for hybrid drive vehicles often maynot be larger than for a vehicle driven only by an internal combustionengine. This means then that a very restricted volume is provided forarranging an electric machine to be a part in the hybrid drive device,so that it is very important that the electric machine is designed sothat a torque per volume unit available being as high as possible isobtained.

In one type of electric machines possible to use in a hybrid drivedevice defined in the introduction the windings of one part, which maybe a rotor or a stator, are achieved by so called distributed windingtechnique, which means that the conductors belonging to different phasesare intermixed. This winding technique makes the coil ends large, i.e.they demand quite some space in radial direction, which results in areduced axial surface for the portions of the first part, such as polepieces of iron, conducting the axial magnetic flux and transmitting thepower between the parts. It is namely the size of the surface of theseportions directed axially that determines how great the torque is thatmay be transferred between the parts, and the torque and the area of thesurface are in a substantially linear relation to each other. In a givenmaximum axial surface allowed to be occupied by said first partincluding the windings, which results from the maximum outer measure ofthe electric machine in the radial direction, the area for the torquetransfer will accordingly be the smaller the more place the coil endsdemand in the radial direction.

Another electric machine already known and possible in a hybrid drivedevice is a so called torus wound machine, which has the additionaldrawback that it, for an optimum utilization of material, requires thatmembers for co-operating by transmitting power through magnetism, suchas permanent magnets, are arranged on both sides of the stator of themachine, so that such an electric machine sometimes may require much toomuch place in the axial direction of the machine.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a hybrid drive deviceof the type defined in the introduction, which solves the problem of theoften small spaces available and still demands of considerable torquesto transfer in such hybrid drive devices.

This object is according to the invention obtained by in such a hybriddrive device the electric machine of the energy converter has said tworotors arranged with surfaces for co-operation by transmitting powerthrough magnetism directed substantially in the direction of the axis ofrotation of the rotors for substantially axial magnetic flux between therotors, that the windings of said one rotor are adapted to be connectedto a multiple phase alternating voltage for generating axial magneticflux, that these rotor windings are arranged in the form of poles beingconcentrically wound, i.e. with the windings intended for connection todifferent phases of said multiple phase alternating voltage separatedfrom each other by being wound around different pole pieces of the rotorand carried out substantially in planes being substantiallyperpendicular to the axis of the rotor.

It is hereby obtained that a considerable torque may be transferredthrough the energy converter of the hybrid drive device in spite of asmall space available therefore. The coil ends of the windings get asmall extension in radial direction, so that the radial extension of thepole pieces conducting the magnetic flux, such as iron, may be madelarger and the area of the axial surface for transferring torque mayhereby be increased, so that the maximum transferable torque at a givenradius of a casing surrounding the electric machine may be increased,i.e. the so called torque density gets higher.

It is here appropriate to mention that the U.S. Pat. No. 5,442,250describes an electric machine having windings for the different phaseswound around separate pole pieces, but it is here a question about amachine with radially directed magnetic flux between a rotor and astator, so that that machine is of another type than the one used in theenergy converter of the hybrid drive device according to the invention.Neither is it a part of a hybrid drive device. Accordingly, there is notthe same problem of obtaining a maximum torque at a given radius as forthe invention. The very goal with that particular division of the phasewindings is instead to simplify the drive electronic of the electricmachine. Furthermore, the windings are there carried out in planes beingsubstantially in parallel with the axis of rotation of the rotor, i.e.substantially perpendicular to the plane in which the windings in theelectric machine according to the invention are carried out.

According to a preferred embodiment of the invention the two rotors areadapted to form a reluctance machine with poles magnetically imprintedon the second of the rotors for increasing the reactance between the tworotors at the poles with respect to between these poles. The advantageobtained by this is that compared with the case of an arrangement ofpermanent magnets the iron losses are eliminated should there be anydesire to achieve a rotation of the second part with respect to anotherpart of the electric machine without transferring any power betweenthese parts in any operation condition. Another advantage of areluctance machine with respect to a permanent magnet machine is that nodetectors of the position of the second part are needed in thereluctance machine case, since it will all the time be adapted to thefirst part.

According to another preferred embodiment of the invention said tworotors are adapted to be able to rotate with different numbers ofrevolutions, and the electric machine of the energy converter comprisesalso a stator adapted to co-operate with one of the rotors bytransmitting power through magnetism. It is then advantageous that oneof the rotors as well as the stator has said windings for co-operationby transmitting power through magnetism, so that a double axial machinewith the winding according to the invention of these two parts and theadvantages associated therewith with respect to a high torque densityand an easy way to cool may be obtained in this way.

According to another preferred embodiment of the invention said windingsare adapted to be connected to a three phase alternating voltage and thenumber of poles of said one rotor is a multiple of three. By this nospecial arrangements for obtaining 120 electrical degrees between thedifferent phases are needed. The number of poles is preferably 6, 9, 12,or 18. An advantage of a lower number of poles is that a lower frequencyof the alternating voltage is required for obtaining a determined numberof revolutions of the electric machine, which keeps the eddy-currentlosses down, which increase with the frequency, but it is then insteadnecessary to renounce the torque density somewhat, since an increasednumber of poles would enable a lower number of winding turns around eachpole and thereby a still smaller radial extension of the coil ends ofthe windings.

According to another preferred embodiment of the invention said windingsare carried out around pole pieces formed by a package of thinsuperposed material layers having a high reactance. Such pole pieces,which are preferably made of electric sheets of iron, results in reducededdy-current losses. The sheets should then extend in substantially thesame plane as the magnetic flux lines therethrough, so that theseexperience a very thin surface. However, the eddy-current losses may beeven more reduced if, according to another preferred embodiment of theinvention, the windings are carried out around pole pieces formed bybound iron powder. It gets in this case possible to accept a higherfrequency of the alternating voltage without having too high losses forthat sake.

According to another preferred embodiment of the invention the energyconverter comprises a unit adapted to be able to influence the torque ofa drive shaft out from the energy converter without changing the numberof revolutions of this shaft or changing the number of revolutions ofthe output shaft of the internal combustion engine, and the devicecomprises a regulating arrangement adapted to co-ordinate control ofenergy flows from the internal combustion engine, to or from theelectric machine and said unit. A hybrid drive device designed in thisway, except that it lacks an electric machine according to the presentinvention, is described in the Swedish patent application 9804261-7 ofthe applicant and has among others the advantage that it is possible toobtain a very high efficiency of the internal combustion engine, sincethe existence of said unit in combination with the electric energyconverter gives an increased freedom to let the internal combustionengine operate at an optimum number of revolutions and torque, whichalso results in a possibility to keep the emissions from the internalcombustion engine on a comparatively low level. Thus, it gets possibleto influence the torque of the drive shaft without changing the numberof revolutions of this shaft, which makes it possible to maintain anunchanged number of revolutions of the output shaft of the internalcombustion engine in spite a change of the torque on the drive shaftwhen desired. Further reasons for designing a hybrid drive device inthis way may be found in said Swedish patent application.

Said drive shaft is advantageously a drive shaft for driving a wheelaxle of a wheeled vehicle, especially a car.

Further advantages as well as advantageous features of the inventionappear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description ofpreferred embodiments of the invention. In the drawings:

FIG. 1 is a partially schematical view illustrating an axial partialsection through an electric machine used in an energy converter in ahybrid drive device according to a first preferred embodiment of theinvention,

FIG. 2 is a detailed view of a part of the machine according to FIG. 1in axial direction,

FIG. 3 is an enlarged view of a portion of the part shown in FIG. 2 inradial direction,

FIG. 4 is a simplified view corresponding to FIG. 1 of an electricmachine used in an energy converter in a hybrid drive device accordingto a second preferred embodiment of the invention,

FIG. 5 is a view corresponding to FIG. 4 of an electric machine used inan energy converter in a hybrid drive device according to a thirdpreferred embodiment of the invention,

FIG. 6 is a view corresponding to FIG. 4 of an electric machine used inan energy converter in a hybrid drive device according to a fourthpreferred embodiment of the invention,

FIG. 7 is a perspective view of the rotor of the electric machineaccording to FIG. 6, and

FIG. 8 is a simplified view of a hybrid drive device according to apreferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

An electric machine used in an energy converter in a hybrid drive deviceaccording to a first preferred embodiment of the invention isschematically illustrated in FIG. 1, and it is also indicated how it maybe built in in a hybrid drive device. It will be explained further belowwith reference to FIG. 8 how the rest of such a hybrid drive device maybe realized and function.

The electric machine has a first rotor 1, which has windings 2schematically indicated applied around pole pieces in a way to beexplained further below with reference to FIGS. 2 and 3. The windings 2are through slip rings 4 running on the shaft 3 of the rotor connectedto an energy source 5 schematically indicated, such as a battery, forreceiving a three phase alternating voltage through a converter 6, forexample a six pulse bridge.

The machine has also a second rotor 7, which is arranged coaxially withrespect to the first rotor and on one side thereof has permanent magnets8 facing towards axially directed surfaces of the first rotor with asmall air gap therebetween for co-operation of the two rotors bytransmitting power through magnetism. The second rotor 7 has on theother side thereof permanent magnets 9 directed oppositely axially andadapted to co-operate with a stator 10 by transmitting power throughmagnetism, said stator being constructed in the same way as the firstrotor 1 with respect to the windings 2 and the connection to the energysource 5 through a converter 12. The rotors 1, 7 are rotatably receivedin bearings not shown while the stator 10 is stationary arranged bybeing fixed to a frame of the machine. The first rotor 1 may for examplebe connected through the shaft 3 thereof with the output shaft of aninternal combustion engine, while the second rotor 7 may through theshaft thereof be connected to a drive shaft for for example wheels of avehicle.

It is also shown how a casing 13 receiving the rotors and the stator isarranged. It is schematically illustrated how cooling loops 14 are builtinto the wall 15 of the casing for cooling those parts of the electricmachine which get hot when transferring torque through magnetic fluxdirected substantially between the first rotor and the second rotor aswell as between the second rotor and the stator. Gases as well as liquidcooling media, for example air or water, may be utilized in the coolingloops 14.

The very construction of the stator 10 will now be explained whilemaking reference to FIGS 2 arid 3, and the first rotor 1 is constructedin the corresponding way. The stator has nine pole pieces 16 in the formof sector-shaped packages of thin sheets 17 (only shown for one polepiece) extending in planes being substantially in parallel with therotation symmetry axis of the stator. It is shown in FIG. 3 how thesector-shaped pole pieces have slots 18 running substantially radiallyfor receiving conductors 19 of the windings 2. The cross section of theslots are designed so that a winding having a sufficient cross sectionfor obtaining a certain current without too high current density may bereceived in each slot.

The windings are concentrically carried out around the poles, i.e. onepole piece has only windings connected to one phase of said alternatingvoltage arranged therearound, so that the windings belonging to adjacentpole pieces are intended to be connected to different phases of thealternating voltage. More exactly, each third pole piece as seen in thecircumferential direction of the stator is provided with windingsconnected to the same alternating voltage phase. This means that thecoil ends 20 of the windings, which are only schematically illustratedfor one pole piece in FIG. 2, may be given a very small extension in theradial direction of the stator, so that the pole pieces instead may begiven a longer extension in the radial direction than if said coil endswould be thicker. The total surface of the pole pieces 16 axiallydirected may by this be increased, and it is the size of this surfacethat determines the amount of torque that may be transferred between thestator and the second rotor. This torque is namely transferred throughthe iron sheets through the magnet field formed therein through thealternating voltage in the windings 2 surrounding these. Through theco-operation of this magnet field with the permanent magnets 9 of thesecond rotor the latter may be driven to rotate.

It is very schematically illustrated in FIG. 3 how magnet field lines 21run through “the iron” 16 for leaving this in a substantially axialdirection and then through the magnets 9 return for being closed in “theiron” 16.

It may as an example be mentioned that the pole width at the middle ofeach pole piece at the air gap may be 56.3 mm, while the thickness ofthe pole piece at the slots at the middle could be 28 mm. The opening 22could then be 3 mm and 15 turns of a conductor with a conductor area of25.5 mm² could then be wound around each pole piece. The inner diameterof the casing 13 could then be 260 mm, while the diameter of the innerring formed by the pole pieces is 120 mm and the outer diameter of theouter ring 220 mm. Thus, the coil ends manage with a radial distance of20 mm between the pole pieces and the casing.

The number of poles of the second rotor 9 is different than of the firstrotor 1 or the stator 10. More exactly, it is in the three phase case ina relation of 2:3, 1:3, 1:4.5 and so on to the number of poles of thefirst rotor 1 and of the stator 10. It is in the present case six.

An electric machine used in an energy converter in a hybrid drive deviceaccording to a second preferred embodiment of the invention is veryschematically shown in FIG. 4 and it differs from the one according toFIG. 1 by not having any stator, which also means that permanent magnetsare only arranged on one side of the second rotor. The parts havingcorrespondence in FIG. 1 have in this figure been provided with the samereference numerals.

An electric machine used in an energy converter in a hybrid drive deviceaccording to a third preferred embodiment of the invention is veryschematically shown in FIG. 5 and it differs from the one according toFIG. 4 by the fact that the first rotor 1 is provided with permanentmagnets and here the second rotor is replaced by the stator 10 havingwindings concentrically arranged around the poles thereof and it formssaid first part. The parts having correspondence in FIG. 1 are in thisfigure provided with the same reference numerals.

An electric machine used in an energy converter in a hybrid drive deviceaccording to a fourth preferred embodiment of the invention is in FIG. 6shown in a view corresponding to FIG. 4, and the parts corresponding toparts of the embodiment according to FIG. 1 are also here provided withthe same reference numerals. There is no second rotor in this electricmachine, but only two parts movable with respect to each other in theform of a first rotor 1 and a stator 10. The stator 10 constitutes herethe first part, i.e. it is provided with the concentrical windingsaround the pole pieces, which are connected to the alternating voltage.Furthermore, this embodiment differs from the previous ones by the factthat the first rotor is provided with poles 23 being magneticallyimprinted, so that the electric machine will be a synchronous reluctancemachine. How this first rotor is designed appears more clearly from FIG.7. Thus, the magnetic imprinting is here a physical imprinting by ashorter air gap of the poles 23 to the pole pieces of a stator thanadjacent recess-like parts 24 of the rotor. However, the magneticimprinting may be achieved by a pure material choice without anyphysical imprinting. The magnetic imprinted poles 23 are designed toincrease the reactance between the stator 10 and the rotor 1 at thepoles 23 with respect to between these poles by a smaller air gap there,so that a current in the stator windings will result in that a magneticflux resulting therefrom (indicated by lines 25) will try to closethrough the iron of the rotor in the most easy way, i.e. where the airgap between the rotor and the stator is smallest, and if one pole isobliquely located with respect to the magnetic flux a torque isgenerated on the rotor, which tries to turn the pole into alignment withrespect to the magnetic flux. This constitutes conventional techniquefor synchronous reluctance machines. The comparison of FIGS. 6 and 7shows that the section in FIG. 6 is carried out in two planes making anangle to each other for illustrating the imprinting.

Finally, the principle of the hybrid drive device according to theinvention and how a preferred embodiment thereof may be constructed isschematically illustrated in FIG. 8. The device comprises an internalcombustion engine 26, which forms an input shaft 27 of an energyconverter 28, which here is formed by an electric machine shown inFIG. 1. How this is constructed has already been described, and this isalso valid for the energy source 5 with the converter 6, 12. It is herepointed out that the converter 6, 12 may also convert alternatingvoltage into direct voltage when feeding energy to the energy source 5(a battery). It is also possible to feed energy between the converters.

The second rotor is connected to an output drive shaft 29, from theenergy converter 28, which could receive a reduction of the number ofrevolutions in a reduction gear and by this transfer the rotation torquethereof to the wheel axle 30 of a wheeled vehicle 31 schematicallyindicated, such as a private car, a lorry or the like. However, a gear32 may instead be arranged between the output shaft of the internalcombustion engine and the first of the rotors, in which this gear thennormally has the task to increase the number of revolutions of theinternal combustion engine for reducing the difference in the number ofrevolutions between the two rotors. However, in the normal case the gear32 will be located “after” the rotor. The device comprises also aregulating arrangement 33 schematically indicated adapted to co-ordinatecontrol of energy flows to or from the internal combustion engine andthe two electric machines.

It is a matter of course that an amount of different possibilities tocombinations of energy flows are possible in such a hybrid drive device,and for a more exact description of the details thereof reference ismade to the Swedish patent application 9804261-7 already mentioned.Thus, the drive shaft 29 may for example be driven through energy fromonly the internal combustion engine, only the battery 5 or both of them.It may as an example be mentioned that when driving with a so called“cruise control”, i.e. with a constant speed when driving on a highway,the regulating arrangement may ensure that locking members not shownconnect the output shaft of the internal combustion engine with thedrive shaft, and the wheel axle is on an even ground drivensubstantially only by the internal combustion engine, and electricalenergy is supplied to the stator from the battery 5 when suddenlydriving uphill for giving a torque addition to the drive shaft andenable a constant speed there without changing the number of revolutionsor the torque of the internal combustion engine. This way of driving ona highway means for the rest a charging of the battery, so that it willthen be possible to disconnect the internal combustion engine and onlydrive with energy from the battery when driving in a city.

An advantage of an electric machine according to the invention in such ahybrid drive device is that a very high torque density may be obtainedthanks to the axial magnetic flux and the way in which the windings arecarried out, so that it will be possible to transfer high torquesdesired on the drive shaft 29 also when the vehicle is driven by energyonly from the battery in spite of the very restricted space availablefor the electric machine.

The invention is of course not in any way restricted to the preferredembodiments described above, but many possibilities to modificationsthereof will be apparent to a person with skill in the art withoutdeparting from the basic idea of the invention as defined in the claims.

The electric machine of the hybrid drive device according to theinvention may for example have another number of parts movable withrespect to each other than shown in the figures, for example more thanthree.

The number of poles of the rotor and/or stator could also be totallydifferent. It is also theoretically conceivable that the number ofphases of the alternating voltage in the windings carried outconcentrically could be another than three, for example two, four orfive, should there be a desire to do so.

1. A hybrid drive device, comprising: an internal combustion engine, anenergy converter adapted to co-operate with an output shaft of theinternal combustion engine and which comprises at least one electricmachine having a first rotor connected to the output shaft of theinternal combustion engine and a second rotor connected to a driveshaft, in which the rotors are adapted to co-operate with each other bytransmitting power through magnetism, in which at least one of therotors is provided with rotor windings, and in which the devicecomprises an energy source adapted to exchange electrical energy withsaid rotor windings through an alternating voltage in said rotorwindings; wherein the electric machine has said two rotors arranged withsurfaces for cooperation by transmitting power through magnetismdirected substantially in the direction of the axis of rotation of therotors for substantially axial magnetic flux between the rotors; andwherein the rotor windings are adapted to be connected to a multiplephase alternating voltage for generating axial magnetic flux, whereinthese rotor windings are arranged in the form of poles beingconcentrically wound, with the rotor windings adapted for connection todifferent phases of said multiple phase alternating voltage separatedfrom each other by being wound around different pole pieces and carriedout substantially in planes substantially perpendicular to said axis ofrotation.
 2. A device according to claim 1, wherein the second rotor isprovided with permanent magnets for said cooperation by transmittingpower through magnetism with said first rotor.
 3. A device according toclaim 1, wherein the first and second rotors are adapted to form areluctance machine with poles magnetically imprinted on the second rotorfor increasing the reactance between the two rotors at the poles withrespect to between these poles.
 4. A device according to claim 1,wherein said rotors are arranged to be able to rotate with differentnumbers of revolutions, and the electric machine of the energy converterfurther comprises a stator adapted to co-operate with one of the rotorsby transmitting power through magnetism.
 5. A device according to claim4, wherein one of the rotors and the stator has said windings forcooperating by transmitting power through magnetism throughsubstantially axial magnetic flux between each of them and the otherrotor.
 6. A device according to claim 1, further comprising a casingextending axially and surrounding said first and second rotors, andmeans for cooling the rotors arranged in the wall of the casing.
 7. Adevice according to claim 1, wherein said rotor windings are adapted tobe connected to a three phase alternating voltage and the number ofpoles of said at least one rotor is a multiple of three.
 8. A deviceaccording to claim 7, wherein the number of poles is 6, 9, 12 or
 18. 9.A device according to claim 1, wherein said windings are carried outaround pole pieces formed by a package of thin superposed materiallayers having a high reluctance.
 10. A device according to claim 1,wherein the windings are carried out around pole pieces formed by boundiron powder.
 11. A device according to claim 1, wherein said windingsare carried out around pole pieces having a substantially circularsector-like shape having each at least one slot extending substantiallyradially for receiving the windings.
 12. A device according to claim 1,wherein the energy converter comprises a unit adapted to be able toinfluence the torque of a drive shaft out from the energy converterwithout changing the number of revolutions of this shaft or changing thenumber of revolutions of the output shaft of the internal combustionengine, and wherein the device further comprises a regulatingarrangement adapted to co-ordinate control of energy flows from theinternal combustion engine, to or from the electric machine and saidunit.
 13. A device according to claim 1, wherein said drive shaft isconfigured for driving a wheel axle of a wheeled vehicle.
 14. Anelectric machine, comprising: a first rotor configured to be connectedto an output shaft of an engine, said first rotor being adapted tocooperate with at least one of a second rotor and a stator bytransmitting power through magnetism, said magnetism being effected byan axial magnetic flux directed substantially in a direction of an axisof rotation of said first rotor; and a plurality of rotor windings, saidrotor windings being adapted to be connected to a multiple phasealternating voltage for generating said axial magnetic flux.
 15. Anelectric machine as recited in claim 14, wherein said rotor windings arearranged in the form of poles concentrically wound on said first rotor,and wherein said at least one of a second rotor and a stator comprises aplurality of permanent magnets configured so as to face said windingsacross an air gap.
 16. An electric machine as recited in claim 14,wherein said windings are arranged in the form of poles concentricallywound on said stator, and wherein said first rotor comprises a pluralityof permanent magnets configured so as to face said stator across an airgap.
 17. An electric machine as recited in claim 14, wherein saidwindings are arranged in the form of poles concentrically wound on saidsecond rotor, and wherein said first rotor comprises a plurality ofpermanent magnets configured so as to face said windings across an airgap.
 18. An electric machine as recited in claim 14, wherein said rotorwindings are adapted for connection to different phases of said multiplephase alternating voltage, and are separated from each other by beingwound around different pole pieces and carried out substantially inplanes being substantially perpendicular to said axis of rotation. 19.An electric machine as recited in claim 14, further comprising an energysource adapted to exchange electrical energy with said windings.
 20. Anelectric machine as recited in claim 19, further comprising a converterdisposed between said energy source and said windings.
 21. An electricmachine as recited in claim 20, wherein said energy source comprises abattery and said converter is capable of converting a voltage output ofsaid battery into said multiple phase alternating voltage.
 22. Anelectric machine as recited in claim 21, wherein said converter isfurther capable of converting an alternating voltage provided by saidwindings into a DC voltage for charging said battery.
 23. An electricmachine as recited in claim 14, comprising said second rotor, saidsecond rotor being configured to be connected to a drive shaft.
 24. Anelectric machine as recited in claim 14, wherein said rotor windings arearranged in the form of a first set of poles concentrically wound onsaid first rotor, and wherein said at least one of a second rotor and astator comprises a plurality of permanent magnets configured so as toface said windings across an air gap, said permanent magnets forming asecond set of poles wherein the number of poles in said first set isequal to the number of poles in said second set.
 25. An electric machineas recited in claim 13, wherein said rotor windings are arranged in theform of a first set of poles concentrically wound on said first rotor,and wherein said at least one of a second rotor and a stator comprises aplurality of permanent magnets configured so as to face said windingsacross an air gap, said permanent magnets forming a second set of poleswherein the number of poles in said first set is different from thenumber of poles in said second set.
 26. An electric machine, comprising:a first rotor configured to be connected to an output shaft of anengine; a second rotor arranged coaxially with the first rotor; and aplurality of rotor windings of the first rotor, said rotor windingsbeing adapted to be connected to a multiple phase alternating voltagefor generating said axial magnetic flux; wherein the first rotor isadapted to cooperate with the second rotor by transmitting power throughmagnetism, said magnetism being effected by an axial magnetic fluxdirected substantially in a direction of an axis of rotation of saidfirst rotor.
 27. An electric machine as recited in claim 26, wherein thesecond rotor is provided with first permanent magnets facing towardsaxially directed surfaces of the first rotor for said cooperation bytransmitting power through magnetism with said first rotor.
 28. Anelectric machine as recited in claim 27, further comprising: a statoradapted to co-operate with one of the rotors by transmitting powerthrough magnetism.
 29. An electric machine as recited in claim 28,wherein the second rotor is located axially intermediate the first rotorand the stator, the second rotor being provided with second permanentmagnets directed axially opposite the first permanent magnet, the secondpermanent magnets adapted to cooperate with the stator by transmittingpower through magnetism.